Embodiments of the present invention relate generally to integrated circuit (IC) interfaces, and more specifically to integrated circuit interfaces having equalization capabilities.
Integrated circuits typically have dedicated interfaces to communicate within and outside of a system. Signals communicated via ICs continue to be transmitted at higher speeds. As these signal speeds increase, the effect of imperfect channels or interconnects through which the signal passes also increases. For example, a channel may be a circuit board trace or a wire routed between integrated circuits. Depending on the characteristics of the interconnect, frequency dependent attenuation of signal amplitude may occur. In general, when the signal amplitude is attenuated as a function of frequency, the signal becomes smaller as the speed increases. When the signal gets too small, communications between devices can become unreliable.
Furthermore, a transmitted symbol may be misinterpreted by a receiver because of lingering effects associated with symbols that were previously transmitted via an interconnect. For example, a significant amount of voltage might remain on the interconnect after a long series of logic one values are transmitted. As a result, a receiver may mistakenly interpret a newly transmitted logic zero value as a logic one. Such problems, referred to as inter-symbol interference (ISI), can limit data rates. ISI can also vary based on frequency dependent insertion loss characteristics of an interconnect, and this interference increases at higher transfer rates.
To reduce these problems, different equalization systems can be associated with an interconnect. As an example, passive equalization networks can be used with an interconnect. Such an approach, however, can introduce additional insertion loss in the link. Moreover, a network topology, and the values of associated components, is highly dependent on the insertion loss characteristics of the particular interconnect (which might not be known at the time the passive equalization network is designed). Similarly, active equalization may be implemented at a transmitter or receiver to equalize the channel. For example, an adaptive-tapped delay line filter may be designed to cancel out frequency dependent loss characteristics of an interconnect. In this case, however, the appropriate filter coefficients will depend on the particular interconnection media and topology, which may change over time, requiring continual adjusting of filter coefficients.
Many equalizers use fixed taps that cannot thereafter be adjusted. This is disadvantageous for a number of reasons. For example, the number of taps and filter coefficient settings for one medium or channel may not be optimal or may not even work with another channel. To overcome these inconsistencies, users may manually vary certain parameters of the filter to make the link work for different channels, taking into consideration bit-rate as well as other variables. However, this is time inefficient, and also undermines system flexibility and adaptability.